This invention relates generally to the field of data storage devices, and more particularly, but not by way of limitation, to incorporation of a method for linearizing the non-linear hysteresis loop of a microactuator for a disc drive.
Disc drives are used for data storage in modem electronic products ranging from digital cameras to computer systems and networks. Typically, disc drive includes a mechanical portion, or head disc assembly (HDA), and electronics in the form of a printed circuit board assembly (PCB), mounted to an outer surface of the HDA. The PCB controls HDA functions and provides an interface between the disc drive and its host.
Generally, a HDA comprises one or more magnetic discs affixed to a spindle motor assembly for rotation at a constant speed, an actuator assembly supporting an array of read/write heads that traverse generally concentric data tracks radially spaced across the disc surfaces and a voice coil motor (VCM) providing rotational motion to the actuator assembly. Modem disc drives typically utilize magneto resistive head technology that employs both an inductive element, for writing data to the data tracks and a magneto resistive element for reading data from the recording tracks.
Continued demand for disc drives with ever increasing levels of data storage capacity, faster data throughput and decreasing price per megabyte have led disc drive manufacturers to seek ways to increase the storage capacity and improve overall operating efficiencies of the disc drive. Present generation disc drives typically achieve aerial bit densities of several gigabits per square centimeter, Gbits/cm2. Increasing recording densities can be achieved by increasing the number of bits stored along each track or bits per inch (BPI), generally requiring improvements in the read/write channel electronics, and/or by increasing the number of tracks per unit width or tracks per inch (TPI), generally requiring improvements in servo control systems.
One approach taken by disc drive manufacturers to improve servo control systems has been through the introduction of dual-stage actuator systems. One such system utilizes an arm mounted bipolar piezoelectric transducer (PZT) operating in parallel with the VCM and driven by a bipolar driver. To date, attempts at expanding utilization of the PZT in disc drive manufacturing processes such as measuring track spacing, or positioning a read/write head during servo track writing for improved track-to-track spacing have achieved only limited success. To effectively incorporate use of the PZT within manufacturing processes, the PZT needs to operate in an open loop mode. The presence of a characteristic non-linear hysteresis loop within the PZT has obstructed use of the PZT for positioning the read/write heads with substantially accuracy, while operating in an open loop mode. Absent feedback from a closed loop control system, positioning of read/write heads by the PZT is substantially accurate at the operating end points of the PZT. To accurately utilize the PZT between its operating end points, the non-linear hysteresis loop within the PZT needs to be overcome.
Therefore, challenges remain and a need persists for techniques of linearizing microactuator hysteresis to advance the utilization of a microactuators in an open loop mode within disc drive manufacturing processes that overcome the constraints present in disc drives with piezoelectric transducer based dual-stage actuator systems.
The present invention provides a method for linearizing microactuator hysteresis for a disc drive by selecting a polynomial equation for modeling the microactuator to provide a linear displacement response of the microactuator responding to an applied voltage, and using a first selected read/write head supported by a first actuator arm of the disc drive to servo on a set-point of a servo track of a first rotatable disc surface of the disc drive while using a second selected read/write head supported by a second actuator arm to write a burst pattern on a second rotatable disc surface of the disc drive, and sequentially servoing on each of a predetermined number of set points with the first selected read/write head while using the second selected read/write head to execute a burst scan of the burst written to the second selected rotatable disc surface for each set-point.
Additionally, the present invention provides a method for linearizing a displacement of a microactuator of a disc drive by choosing a second polynomial equation for modeling a predetermined linear displacement of the microactuator and utilizing a set of displacement variables derived from resolving the first polynomial equation based on a set of predetermined voltage values and constants derived from burst scans for resolving the second polynomial equation to provide a set of equations from which a set of constants can be derived for use by the second polynomial equation to displace the microactuator in response to a predetermined incremental increase in voltage applied to the microactuator by utilizing the set of constants and the second polynomial equation to determine a number of digital to analog conversion counts to apply to a microactuator driver driving the microactuator to attain a desired linear displacement of microactuator.
These and various other features and advantages, which characterize embodiments of the present invention, will be apparent from the reading of the following detailed description and review of the associated drawings.